1. Field of the Invention
The present invention relates to a composite ultrasonic transducer formed by regularly arranging a plurality of piezoelectric ceramic columns in a resin plate. Such a composite ultrasonic transducer is applicable to a medical ultrasonic diagnostic apparatus and an industrial nondestructive inspection apparatus.
2. Description of the Background Art
A piezoelectric ceramic plate has been utilized for a long time as an ultrasonic transducer. However, the piezoelectric ceramic plate has an acoustic impedance of approximately 30 MRayl which is much higher than an acoustic impedance of approximately 1.5 MRayl of any biological object, and therefore has a low efficiency of transmitting ultrasonic waves from the piezoelectric ceramic plate to the biological object. In addition, compared with piezoelectric resin such as polyvinyliden fluoride, the piezoelectric ceramic plate has a low efficiency in receiving an ultrasonic signal to convert it to an electric signal while having a high efficiency of converting an electric signal to an ultrasonic signal. In view of these problems, a composite ultrasonic transducer formed of a resin plate including an array of a plurality of small piezoelectric ceramic columns has been proposed and studied (see IEEE Trans. Sonics Ultrasonics, Vol. SU-32, 1985, pp. 481-497).
A composite ultrasonic transducer initially was fabricated by arranging piezoelectric ceramic columns each having a circular shape in a cross section plane perpendicular to a longitudinal axis and filling the space between those ceramic columns with resin. The piezoelectric ceramic columns each had a cross-sectional diameter of at least approximately 300 .mu.m. It is known that various characteristics of the composite ultrasonic transducer depend on the dimension of the piezoelectric ceramic column and the frequency of the ultrasonic wave. For example, if the composite ultrasonic transducer is used in a higher frequency range, piezoelectric ceramic columns each having a smaller cross-sectional area should be used in view of the sensitivity characteristic. Owing to such circumstances, in the field of the medical ultrasonic diagnostic art using ultrasonic waves in the frequency range of at least 2.5 MHz, the composite ultrasonic transducer including the array of piezoelectric ceramic columns each having the cross-sectional area of 300 .mu.m or more is not employed.
In the field of the semiconductor art around 1980, a dicing technique using a diamond saw to cut a silicon substrate began to be employed. The dicing technique was also utilized for fabricating a composite ultrasonic transducer which can be used in the frequency range of 2.5 MHz or more.
For example, according to Japanese Patent Laying-Open No. 58-22046, a piezoelectric ceramic plate is first adhered onto a ferrite substrate, and the ceramic plate is laterally and vertically cut with a pitch of 300 .mu.m using the dicing technique. Consequently, a plurality of piezoelectric ceramic columns each having a square cross-section of approximately 150 .mu.m.times.150 .mu.m are arrayed on the ferrite substrate at positions corresponding to nodes of a square network (hereinafter referred to as "square network array"). Cut grooves between the piezoelectric ceramic columns are filled with a resin layer and thereafter the resin layer and the plurality of piezoelectric ceramic columns are separated from the ferrite substrate to form a plate-like composite ultrasonic transducer as schematically illustrated in the plan view of FIG. 4A and the side view of FIG. 4B. Specifically, a plurality of fine piezoelectric ceramic columns 2 each having the square cross section are arrayed in the square network in a resin plate 3 in a composite ultrasonic transducer 1.
A problem of composite ultrasonic transducer 1 is that an undesirable lateral mode of high-frequency resonance occurs in a direction parallel to a major surface of plate-like transducer 1, while a desired vertical mode of ultrasonic oscillation in a direction of the thickness of transducer 1 is generated. If the lateral mode resonance occurs in a frequency range close to a frequency band of the vertical mode ultrasonic oscillation used, for example, for the ultrasonic diagnosis, the lateral mode resonance accelerates the lateral spreading of ultrasonic waves generated by the vertical mode resonance, leading to a reduction of the resolution of an ultrasonic image. In order to avoid the reduction of the resolution, a central frequency used for the diagnosis is limited to half the lateral mode resonance frequency or less. The resolution of the ultrasonic image is also reduced by a reduction of the frequency of the employed ultrasonic waves.
Generally, the frequency of the lateral mode resonance of the composite ultrasonic transducer is inversely proportional to the pitch of the array of the piezoelectric ceramic columns. Therefore, the array pitch may be made finer in order to increase the frequency of the lateral mode resonance. In composite ultrasonic transducer 1 as illustrated in FIGS. 4A and 4B, one arbitrary side of one arbitrary piezoelectric ceramic column 2 having the square cross-section faces parallel to one side of another ceramic column located closest to the one arbitrary ceramic column. It is considered that the lateral mode resonance is likely to occur due to the interaction between the sides facing in parallel to each other and close to each other.
With such circumstances and progress in the x-ray lithography art, Japanese Patent Laying-Open No. 4-232425 (U.S. Pat. No. 5,164,920) proposes a composite ultrasonic transducer as shown in FIG. 6 fabricated using x-ray lithography. Referring to the perspective view of FIG. 6, a composite ultrasonic transducer 1a includes a plurality of tapered piezoelectric ceramic columns 2a regularly arranged in a resin plate 3a. Specifically, each of tapered piezoelectric ceramic columns 2a has a trapezoidal shape by a longitudinal cross-section plane including a longitudinal central axis, and has a hexagonal shape at a cross-section perpendicular to the central axis.
Each of the piezoelectric ceramic columns 2a is formed to have the hexagonal cross-section in order to densely arrange ceramic columns 2a in resin plate 3a. Each of the piezoelectric ceramic columns 2a is tapered in order to allow one side of a first arbitrary ceramic column 2a having the hexagonal cross-section to face with an angle twice the taper angle toward one side of a second ceramic column located closest to the first ceramic column without being arranged parallel thereto. In other words, those sides of adjacent ones of the ceramic columns facing closest to each other are not parallel to each other so that the interaction between those sides decreases and thus the undesirable lateral mode resonance is considered to be suppressed.
It is considered that, as the taper angle of the piezoelectric ceramic columns 2a is made larger, the undesirable lateral mode resonance could be suppressed more effectively. However, if the taper angle is made too large, the desired vertical oscillation mode in the longitudinal direction of piezoelectric ceramic columns 2a could become non-uniform. Further, even if x-ray lithography is used, it would be difficult to form fine piezoelectric ceramic column 2a having precisely controlled taper angle and hexagonal cross-section.